The invention relates to devices and methods for encoding data into optical trains, and in particular to encoding data into short pulse soliton trains suitable for transmission of data.
High quality continuous tunable microwave/millimeter wave radiation, generated by compact semiconductor lasers, has numerous potential applications, such as dense wavelength division multiplexing, optical short pulse generation and high-speed wireless applications. One of the ways to generate the microwave/millimeter radiation is to use the beating of the two modes of a dual-wavelength source. Most existing dual-mode semiconductor lasers use either physically separated gain sections as shown, e.g. in publication by K. O. Nyairo, I. H. White, C. J. Armistead, and P. A. Kirkby xe2x80x9cMultiple channel signal generation using multichannel grating cavity laser with crossstalk compensationxe2x80x9d, Electron. Letters, vol. 28, pp. 261-263, 1992, or a common gain medium which is shared by the two wavelengths as shown, e.g. in publication by C. L. Wang and C. L. Pan xe2x80x9cTunable dual-wavelength operation of a diode array with an external grating-loaded cavityxe2x80x9d, Applied Physics Letters, vol. 64, pp. 3089-3091, 1994.
Although dual-mode lasers are capable of generating ultra-high beat frequencies of up to several THz, a major challenge is how to encode information data into the optical train. Up to date, the most reliable way of encoding data into fiber optics systems is to use high speed modulators, with electro-optic (EO) modulators being the most common choice. However, the maximum modulation bandwidth of state-of-the-art electro-optic modulators is currently limited to approximately 50 Gb/s. It means that soliton trains having higher repetition rates can not be handled by existing types of modulators. As a result, the speed of data transmission is unavoidably limited, restricting performance of optical transmission systems.
Accordingly, there is a need in the industry for practical and reliable devices and methods for encoding data into high speed optical trains, which would allow to utilize existing types of modulators without limiting speeds of data transmission while being upgradable as the advances in the modulator technology occur.
It is an object of the present invention to provide a method and device for encoding data into a high speed optical train which would avoid the afore-mentioned problems.
Thus, according to one aspect of the present invention there is provided a device for encoding data into high speed optical train, comprising:
means for forming N short pulse optical trains, each of frequency f, carrying encoded data;
means for providing phase shifts between said N trains so as, when the trains are combined, to form one combined optical pulse train of frequency Nf;
means for combining said N trains into said combined optical train.
Preferably, the means for forming N short pulse trains further comprises N data encoding branches, each branch including:
a dual mode laser generating a signal at frequency f defined by the beat frequency between the dual modes;
an optical compressor disposed to receive the laser signal and to compress the duration thereof to form the short pulse train; and
encoding means providing that required data is encoded into the train.
Conveniently, the means for providing phase shifts between said N trains comprises:
means for subharmonic modulation of each laser at frequency f/n, wherein n is an integer, to provide phase locking;
variable delay lines disposed to introduce said phase shifts between the signals.
Advantageously, the means for providing phase shifts further comprises a feedback means for sending control signals to the variable delay lines to adjust the phase shifts so as to ensure that N optical trains interleave in a precise timing.
Preferably, the encoding means is disposed so as to encode data into the laser signal before the signal is compressed in the optical compressor. Alternatively, it may be disposed so as to encode data into the laser signal after the signal is compressed in the optical compressor. The former arrangement has a potential advantage of allowing integration of the encoding means with dual mode lasers into a compact packaging. The means for providing phase shifts between the laser branches may be disposed to provide one of the following configurations: to be disposed so as to introduce phase shifts between the laser signals before encoding and before compressing the signals; or to introduce the phase shifts after encoding and before compressing the signals; or to introduce the phase shifts after compressing the signals and before encoding them with data; or to introduce the phase shifts between the signals after compressing after encoding the signals. Conveniently, the variable delay lines are disposed so as to receive the subharmonically modulated signals. Alternatively, it may be arranged that the means for subharmonic modulation receive laser signals after they are put through the variable delay lines.
Preferably, the dual mode laser is a DFB laser. To provide high quality of dual-mode operation, the dual mode laser comprises first and second DFB sections including one of the gain coupled and loss coupled DFB lasers. The active medium of the DFB laser includes a multiple quantum well structure. The first and second sections include first and second gratings correspondingly, the gratings being formed by etching grooves directly through the multiple quantum well structure. Each grating has a period comprising a first portion and a second portion, with substantially all quantum wells being etched away from the second portion, thus providing no substantial photon emission in the second portion and ensuring no substantial interaction between the laser sections.
Advantageously, the beat frequency f is from about several tens GHz to about several hundred GHz. Depending on the system requirements and the available frequency bandwidth of EO modulators, it may be narrowed to the range from about 20 GHz to about 80 GHz. The duration of pulses in pulse trains is preferably within a range from sub picoseconds to picoseconds.
Preferably, the optical compressor comprises an erbium doped fiber amplifier, followed by a piece of fiber, e.g. dispersion decreasing fiber, dispersion shifted fiber, single mode fiber, or combination thereof.
Conveniently, the encoding means comprises an external modulator. Preferably, the modulator is an electro-optical modulator, e.g. Mach-Zehnder modulator or travelling wave modulator. Conveniently, the means for combining the trains comprises fiber coupler, and the feedback means comprises a narrow bandwidth photodiode and a narrow band electrical amplifier centralized at frequency Nf.
The device may include required number of laser branches, e.g. two, three, four to ten, or any other reasonable number of lasers capable of being combined into one optical train at a required accuracy.
According to another aspect of the invention, there is provided a method for encoding data into high speed optical train, comprising the steps of:
forming N short pulse optical trains, each of frequency f, carrying encoded data;
providing phase shifts between said N trains so as, when the trains are combined, to form one combined optical pulse train of frequency Nf;
combining said N trains into said combined optical train.
Preferably, the step of forming N short pulse trains comprises the step of forming each of the N trains which includes:
providing a dual mode laser generating a signal at frequency f defined by the beat frequency between the dual modes;
compressing the signal in the optical compressor to form the short pulse train; and
providing that required data is encoded into the train.
Advantageously, the step of providing phase shifts between said N trains comprises:
modulating each laser at frequency f/n, which is subharmonic to the beat frequency, to provide phase locking;
forming variable delay lines disposed to introduce said phase shifts between the signals.
Conveniently, the step of providing phase shifts further comprises providing feedback by sending feedback signals to the variable delay lines to adjust the phase shifts so as to ensure that N optical trains interleave in a precise timing. The step of providing that the data is encoded into the train may comprises encoding data into the laser signals before the step of compressing, or alternatively encoding data into the train after the step of compressing.
The method and device described above have the following advantages. Since the separation between the modes in the dual-mode laser can be made within a large range, the optical system employing such data-encoding device is, therefore, very flexible and versatile. Additionally, it can be easily upgraded to a higher bitrate system without the need to modify the system architecture, being largely depending on the available bandwidth of existing modulators and the number of the modulators used. The invented device can be successfully used in tera-bit time domain multiplexed systems (TDM) in optical fibers.